Means and Method for Assessing the Geometry of a Subterranean Fracture During or After a Hydraulic Fracturing Treatment

ABSTRACT

The present invention relates to methods of fracturing a subterranean formation including the step of pumping at least one device actively transmitting data that provide information on the device position, and further comprising the step of assessing the fracture geometry based on the positions of said at least one device or pumping metallic elements, preferably as proppant agents, and further locating the position of said metallic elements with a tool selected from the group consisting of magnetometers, resistivity tools, electromagnetic devices and ultra-long arrays of electrodes. The invention allows monitoring of the fracture geometry and proppant placement.

BACKGROUND OF INVENTION

[0001] This invention relates generally to the art of hydraulicfracturing in subterranean formations and more particularly to a methodand means for assessing the fracture geometry during or after thehydraulic fracturing.

[0002] Hydraulic fracturing is a primary tool for improving wellproductivity by placing or extending cracks or channels from thewellbore to the reservoir. This operation is essentially performed byhydraulically injecting a fracturing fluid into a wellbore penetrating asubterranean formation and forcing the fracturing fluid against theformation strata by pressure. The formation strata or rock is forced tocrack, creating or enlarging one or more fractures. Proppant is placedin the fracture to prevent the fracture from closing and thus, provideimproved flow of the recoverable fluid, i.e., oil, gas or water.

[0003] The proppant is thus used to hold the walls of the fracture apartto create a conductive path to the wellbore after pumping has stopped.Placing the appropriate proppant at the appropriate concentration toform a suitable proppant pack is thus critical to the success of ahydraulic fracture treatment.

[0004] The geometry of the hydraulic fracture placed affects directlythe efficiency of the process and the success of the operation. Thisgeometry is generally inferred using models and data interpretation, butto date, no direct measurements are available. The present invention isaimed at obtaining more direct measurements of the fracture geometry(e.g. length, height away from the wellbore).

[0005] The fracture geometry is often inferred through use of models andinterpretation of pressure measurements. Occasionally, temperature logsand/or radioactive tracer logs are used to infer fracture height nearthe wellbore. Microseismic events generated in the vicinity of thecreated hydraulic fracture are recorded and interpreted to indicate thedirection (azimuth) and length and height of the created fracture.

[0006] However, these known methods are indirect measurement, and relyon interpretations that may be erroneous, and are difficult to use forreal-time evaluation and optimization of the hydraulic fracturetreatment.

[0007] It is therefore an object of the present invention to provide anew approach to evaluate the fracture geometry.

SUMMARY OF INVENTION

[0008] According to the present invention, the fracture geometry isevaluated by placing inside the fracture small devices that, eitheractively or passively, give us measurements of the fracture geometry.Fracture materials (small objects with distinctive properties e.g. metalbeads with very low resistivity) or devices (e.g. small electronic oracoustic transmitters) are introduced into the fracture during thefracture treatment with the fracturing fluid.

[0009] According to a first embodiment of the present invention, activedevices are added into the fracturing fluid. These devices will activelytransmit data that provide information on the device position andthereafter, can be associated with fracture geometry.

[0010] According to another embodiment of the present invention, passivedevices are added into the fracturing fluid. In the preferredembodiment, these passive devices are also used as proppant.

DETAILED DESCRIPTION

[0011] Examples of “active” device include electronic microsensors, forexample such as radio frequency transmitter, or acoustic transceivers.These “active” devices will be integrated with location trackinghardware to transmit their position as they flow with the fracturefluid/slurry inside the created fracture. The microsensors can be pumpedwith the hydraulic fracturing fluids throughout the treatment or duringselected strategic stage of the fracturing treatment (pad, forwardportion of the proppant-loaded fluid, tail portion of theproppant-loaded fluid) to provide direct indication of the fracturelength and height. The microsensors would form a network using wirelesslinks to neighboring microsensors and have location and positioningcapability through for example local positioning algorithms.

[0012] Pressure and Temperature sensors could also be integrated withthe above-mentioned active devices. The resulting pressure andtemperature measurements would be used to better calibrate and advancethe modeling techniques for hydraulic fracture propagation. They wouldalso allow optimization of the fracturing fluids by indicating theactual conditions under which these fluids are expected to perform. Inaddition chemical sensors could also be integrated to allow monitoringof the fluid performance during the treatment.

[0013] Since the number of active devices required is small compared tothe number of proppant grains, it is possible to use devicessignificantly bigger than the proppant pumped in the fracturing fluid.The active devices could be added after the blending unit and slurrypump, for instance through a lateral by-pass.

[0014] Examples of such device include small wireless sensor networksthat combine microsensor technology, low power distributed signalprocessing, and low cost wireless networking capability in a compactsystem as disclosed for instance in International Patent ApplicationWO0126334, preferably using a data-handling protocol such as TinyOS, sothat the devices organize themselves in a network by listening to oneanother, therefore allowing communication from the tip of the fractureto the well and on to the surface even if the signals are weak so thatthe signals are relayed from the farthest devices towards the devicesstill closest to the recorder to allow uninterrupted transmission andcapture of data. The sensors may be designed using MEMS technology orthe spherical shaped semiconductor integrated circuit as known form U.S.

[0015] A recorder placed at surface or, downhole in the wellbore, couldcapture and record/transmit the data sent by the devices to a computerfor further processing and analysis. The data could also be transmittedto offices in any part of the world using the Internet to allow remoteparticipation in decisions affecting the hydraulic fracturing treatmentoutcome.

[0016] Should the frequency range utilized by the electronictransmitters be such that the borehole metal casing would block itstransmission from the formation behind the casing into the wellbore,antennas could be deployed across the perforation tunnels. Theseantennas could be mounted on non-conductive spherical or ovoid ballsslightly larger than the perforation diameter and designed to be pumpedand to seat in some of the perforations and relay the signals across themetallic casing wall. An alternative method of deployment would be forthe transmitter to trail an antenna wire while being pumped.

[0017] A further variant would cover the case where the measuringdevices are optical fibers with a physical link to a recorder at surfaceor in the borehole that would be deployed through the perforations whenthe well is cased perforated or directly into the fracture in an openhole situation. The optical fiber would allow length measurements aswell as pressure and temperature.

[0018] An important alternative embodiment of this invention covers theuse of materials with specific properties that would enable informationon the fracture geometry to be obtained using an additional measurementdevice.

[0019] Specific examples of “passive” materials include the use ofmetallic fibers or beads as proppant. These would replace some or all ofthe conventional proppant and may have sufficient compressive-strengthto resist crushing at fracture closure. A tool to measure resistivity atvarying depths of investigation would be deployed in the borehole of thefractured well. As the proppant is conductive with a significantcontrast in resistivity compared to the surrounding formations, theresistance measurements would be interpreted to provide information onfracture geometry.

[0020] Another example is the use of ferrous/magnetic fibers or beads.These would replace some or all of the conventional proppant and mayhave sufficient compressive strength to resist crushing at fractureclosure. A tool containing magnetometers would be deployed in theborehole of the fractured well. As the proppant generates a significantcontrast in magnetic field compared to the surrounding formations, themagnetic field measurements would be interpreted to provide informationon fracture geometry. According to a variant of this example, themeasuring tools are deployed on the surface or in offset wells. Moregenerally, tools such as resistivity tools, electromagnetic devices, andultra long arrays of electrodes, can easily detect this proppantenabling fracture height, fracture width, and with processing, thepropped fracture length to some extent can be determined.

[0021] A further step is covered whereby the information provided be thetechniques described above would be used to calibrate parameters in afracture propagation model to allow more accurate design andimplementation of fractures in nearby wells in geological formationswith similar properties and immediate action on the design of thefracture being placed to further the economic outcome.

[0022] For example, if the measurements indicate that the fracturetreatment is confined to only a portion of the formation interval beingtreated, real time design tools would validate suggested actions, e.g.increase rate and viscosity of the fluid or use of ball sealer to divertthe fluid and treat the remainder of the interval of interest.

[0023] If the measurements indicate that the sought after tip screenoutdid not occur yet in a typical Frac and Pack treatment and that thefracture created is still at a safe distance from a nearby water zone,the real time design tool would be re-calibrated and used to validate anextension of the pump schedule. This extension would incorporateinjection of additional proppant laden slurry to achieve the tipscreenout necessary for production performance, while not breakingthrough into the water zone.

[0024] The measurements would also indicate the success of specialmaterials and pumping procedures that are utilized during a fracturetreatment to keep the fracture confined away from a nearby water or gaszone. This knowledge would allow either proceeding with the treatmentwith confidence of its economic success, or taking additional actions,e.g. re-design or repeat the special pumping procedure and materials toensure better success at staying away from the water zone.

[0025] Among the “passive” materials, metallic particles may be used.These particles may be added as a “filler” to the proppant or replacespart of the proppant, In a most preferred embodiment, metallic particlesconsisting of an elongated particulate metallic material, whereinindividual particles of said particulate material have a shape with alengthaspect ration greater than 5 are used both as proppant and“passive” materials.

[0026] Advantageously, the use of metallic fibers as proppantcontributes to enhance proppant conductivity and is further compatiblewith techniques known to enhance proppant conductivity such as the useof conductivity enhancing materials (in particular the use of breakers)and the use of non-damaging fracturing based fluids such as gelled oils,viscoelastic surfactant based fluids, foamed fluids and emulsifiedfluids.

[0027] Where at least part of the proppant consists of metallic In allembodiments of the disclosed invention, at least part of the fracturingfluid comprises a proppant essentially consisting essentially of anelongated particulate metallic material, said individual particles ofsaid particulate material have a shape with a lengthaspect rationgreater than 5. Though the elongated material is most commonly a wiresegment, other shapes such as ribbon or fibers having a non-constantdiameter may also be used, provided that the length to equivalentdiameter is greater than 5, preferably greater than 8 and mostpreferably greater than 10. According to a preferred embodiment, theindividual particles of said particulate material have a length rangingbetween about 1 mm and 25 mm, most preferably ranging between about 2 mmand about 15 mm, most preferably from about 5 mm to about 10 mm.Preferred diameters (or equivalent diameter where the base is notcircular) typically range between about 0.1 mm and about 1 mm and mostpreferably between about 0.2 mm and about 0.5 mm. It must be understoodthat depending on the process of manufacturing, small variations ofshapes, lengths and diameters are normally expected.

[0028] The elongated material is substantially metallic but can includean organic part for instance such as a resin-coating. Preferred metalincludes iron, ferrite, low carbon steel, stainless steel andiron-alloys. Depending on the application, and more particularly of theclosure stress expected to be encountered in the fracture, “soft” alloysmay be used though metallic wires having a hardness between about 45 andabout 55 Rockwell C are usually preferred.

[0029] The wire-proppant of the invention can be used during the wholepropping stage or to only prop part of the fracture. In one embodiment,the method of propping a fracture in a subterranean formation comprisestwo non-simultaneous steps of placing a first proppant consisting of anessentially spherical particulate non-metallic material and placing asecond proppant consisting essentially of an elongated material having alength to equivalent diameter greater than 5. By essentially sphericalparticulate non-metallic material it is meant hereby any conventionalproppant, well known from those skilled in the art of fracturing, andconsisting for instance of sand, silica, synthetic organic particles,glass microspheres, ceramics including alumino-silicates, sinteredbauxite and mixtures thereof or deformable particulate material asdescribed for instance in U.S. Pat. No. 6,330,916. In anotherembodiment, the wire-proppant is only added to a portion of thefracturing fluid, preferably the tail portion. In both cases, thewire-proppant of the invention is not blended with the conventionalmaterial and the fracture proppant material or if blended with, theconventional material makes up to no more than about 25% by weight ofthe total fracture proppant mixture, preferably no more than about 15%by weight.

[0030] Experiemental Methods:

[0031] A test was made to compare proppant made of metallic balls, madeof stainless steel SS 302, having an average diameter of about 1.6 mmand wire proppant manufactured by cutting an uncoated iron wire of SS302 stainless steel into segments approximately 7.6 mm long. The wirewas about 1.6 mm diameter.

[0032] The proppant was deposited between two Ohio sandstone slabs in afracture conductivity apparatus and subjected to a standard proppantpack conductivity test. The experiments were done at 100 Â° F., 2 lb/ftproppant loading and 3 closure stresses, 3000, 6000 and 9000 psi(corresponding to about 20.6, 41.4 and 62 MPa). The permeability,fracture gap and conductivity results of steel balls and wires are shownin Table 1. Permeability · Fracture · Conductivity

losure · Str- (darcy)

Gap

· (inch)

(md-ft)

ess¶ (psi)

Ball

Wire

Ball

Wire

Ball

Wire

3000

3,703

10,335

0.085

0.119

26,232

102,398

6000

1,077

4,126

0.061

0.095

5,472

33,090

9000

705

1,304

0.064

0.076

3,174

8,249

[0033] The conductivity is the product of the permeability (inmilliDarcy) by the fracture gap (in feet).

1. A method of fracturing a subterranean formation comprising injectinga fracturing fluid, into a hydraulic fracture created into asubterranean formation, wherein at least a portion of the fracturingfluid comprises at least one device actively transmitting data thatprovide information on the device position, and further comprising thestep of assessing the fracture geometry based on the positions of saiddevices.
 2. The method of claim 1, wherein said devices are electronicdevices.
 3. The method of claim 2, wherein said devices are radiofrequency or other EM wave transmitters.
 4. The method of claim 1,wherein said devices are—acoustic devices.
 5. The method of claim 4,wherein said devices are ultrasonic transceivers.
 6. The method of claim1, wherein at least one device is pumped during the pad stage and atleast one device is pumped during the tail portion.
 7. The method ofclaim 1, wherein said devices also transmit information as to thetemperature of the surrounding formation.
 8. The method of claim 1,wherein said devices also transmit information as to the pressure. 9.The method of claim 1, wherein a plurality of devices is injected, saiddevices organized in a wireless network.
 10. The method of claim 1,wherein the devices are electronic transmitters and the method furtherincludes the deployment of at least an antenna.
 11. The method of claim10, wherein antennas are mounted on non-conductive balls that are pumpedwith the fluid and seat in some of the perforations relaying the signalsfrom sensors behind the casing wall.
 12. The method of claim 10, whereinthe antenna is trailed by the transmitter within the fracture while thetransmitter is pumped.
 13. The method of claim 1, where the device is anoptical fiber deployed through the perforation.
 14. The method of claim13, wherein the optical fiber is further deployed through the fracture.15. A method of fracturing a subterranean formation comprising injectinga fracturing fluid, into a hydraulic fracture created into asubterranean formation, wherein at least a portion of the fracturingfluid comprises metallic elements and further comprising the step oflocating the position of said metallic elements with a tool selectedfrom the group consisting of magnetometers, resistivity tools,electromagnetic devices and ultra-long arrays of electrodes.
 16. Themethod of claim 15 wherein said metallic material comprises elongatedparticles having a length to equivalent diameter greater than
 5. 17. Themethod of claim 16, wherein said particles have a shape with alengthaspect ration greater than
 10. 18. The method of claim 16, whereinsaid elongated particles have a wire-segment shape.
 19. The method ofclaim 16, wherein said elongated particles are in a material selectedfrom the group consisting of iron, ferrite, low carbon steel, stainlesssteel and iron-alloys.
 20. The method of claim 16, where said elongatedparticles consists of metallic wires having a hardness of between 45 and55 Rockwell.
 21. The method of claim 16, wherein said elongatedparticles are resin-coated.
 22. The method of claim 16, wherein saidelongated particles have a length of between 1 and 25 mm.
 23. The methodof claim 22, wherein said elongated particles have a length of betweenabout 2 and about 15 mm.
 24. The method of claim 16, wherein saidelongated particles have a diameter of between about 0.1 mm and about 1mm.
 25. The method of claim 16, wherein said individual particles ofsaid elongated particulate material have a diameter of between about 0.2mm and about 0.5 mm.
 26. The method of claim 1, wherein the geometry ofthe fracture is monitored in real-time during the hydraulic fracturingtreatment.
 27. The method of claim 15, wherein the geometry of thefracture is monitored in real-time during the hydraulic fracturingtreatment.